1. Field of the Invention
The present invention relates to a power supply circuit and to a control method for the same.
2. Description of the Related Art
In a liquid crystal display device, for example, a scanning line drive circuit and a signal line drive circuit are used to drive a liquid crystal panel having scanning lines and signal lines arrayed in a matrix. The scanning line drive circuit drives scanning on the scanning lines and the signal line drive circuit applies data signals to the signal lines. The scanning line and signal line drivers operate synchronously in order to display something on the liquid crystal panel. The display timing and operation of the scanning and signal line drive circuits are controlled by a liquid crystal display (LCD) controller (also simply called a display controller).
The drive voltage of the scanning line drive circuit and the signal line drive circuit differ, however, depending upon the liquid crystal material used in the liquid crystal panel and the LCD driving method. Therefore, because the scanning line drive circuit and signal line drive circuit are manufactured in processes with a voltage resistance determined by the drive voltage, the drive voltage supplied to each may differ. The display controller integrates complex circuitry, and may therefore be manufactured in a low voltage process.
An LCD therefore has a power supply circuit to supply the appropriate supply voltage to the various circuits driving the liquid crystal panel. The power supply circuit generates these different voltages based on an externally supplied system power supply.
One type of power supply circuit has a charge pump type step-up/step-down circuit as the circuit for generating the various voltages. A charge pump type step-up/step-down circuit can generate voltage stepped up or down in the positive or negative direction with high efficiency and low power consumption as a result of charge pumping using switching elements. The charge pump type step-up/step-down circuit needs a switching control signal (step-up clock, step-down clock) to control the switching elements. If this switching control signal is supplied from the display controller, the user can supply a switching control signal at a frequency at which optimum step-up or step-down efficiency can be achieved.
When the system power supply turns on, however, the display controller cannot generate this switching control signal because the appropriate supply voltage is not supplied from the power supply circuit. Furthermore, because the switching control signal is not supplied, the charge pump type step-up/step-down circuit of the power supply circuit cannot generate the appropriate voltage. A series regulator is therefore used to generate the supply voltage in order to accelerate system startup after the power supply is turned on. A problem with using a series regulator, however, is that efficiency is low and power consumption increases.
The present invention is directed to solving these technical problems. Accordingly, an object of the invention is to provide a power supply circuit and control method enabling quick startup after the power supply is turned on while also providing high efficiency and low power consumption.
To achieve these objects, a power supply circuit for generating a third potential based on a first potential and a second potential is provided. According to one aspect of the invention, the power supply comprises a step-down circuit for generating, based on a switching control signal, a step-down potential by stepping down the second potential using a charge pumping operation; a first potential adjusting circuit for generating an adjusted potential by adjusting the second potential; and a switching circuit for selecting and outputting as the third potential either the step-down potential or the adjusted potential based on a switching signal.
Such a power supply circuit advantageously has both high efficiency and low power consumption provided by the step-down circuit and high drive capacity provided by the first potential adjusting circuit.
Preferably, the step-down circuit comprises a second potential adjusting circuit for adjusting the step-down potential generated by stepping down the second potential. As a result of the second potential adjusting circuit adjusting the potential of the step-down potential, this power supply circuit can supply the third potential as a stable power supply.
Preferably, the switching signal changes after a specific period of time following turn on of the power supply, and the switching circuit switches from outputting the adjusted potential as the third potential to outputting the step-down potential as the third potential when the switching signal changes. This accelerates startup in the period just after the system power supply turns on, and once the system stabilizes makes it possible to supply power with high efficiency and low power consumption.
According to another aspect, the power supply circuit comprises a step-down circuit for generating, based on a switching control signal, a step-down potential by stepping down the second potential using a charge pumping operation; a switching circuit for selecting and outputting as a fourth potential either the step-down potential or the second potential based on a switching signal; and a potential adjusting circuit for adjusting the fourth potential to the third potential.
In this power supply circuit the potential adjusting circuit adjusts the fourth potential, which was selected from either the step-down potential or the second potential, to output the third potential. It is therefore possible to provide a power supply circuit able to advantageously utilize both high efficiency and low power consumption provided by the step-down circuit and high drive capacity of the potential adjusting circuit. Furthermore, because the potential adjusting circuit is located after potential selection, the size of the circuit can be reduced compared with a configuration in which potential adjustment is disposed before potential selection and output.
Preferably, the specific switching signal changes after a specific period of time following turn on of the power supply, and the switching circuit switches from outputting the second potential to outputting the step-down potential to when the switching signal changes. Thus comprised, the second potential is potential adjusted and output as the third potential until the specified period ends; then and after the specific period ends, the potential of the step-down potential is adjusted and output as the third potential. This accelerates startup in the period just after the system power supply turns on, and once the system stabilizes makes it possible to supply power with high efficiency and low power consumption. Furthermore, because the potential adjusting circuit is located after potential selection, circuit scale can be reduced compared with a configuration in which potential adjustment is before potential selection and output.
Preferably, the switching signal changes according to a result of a monitoring circuit monitoring the step-down potential or the third potential. This power supply circuit controls potential selection based on the condition of the step-down potential or the third potential, and can therefore supply power while optimizing control with respect to drive capacity, efficiency, and power consumption.
Preferably, at least the third potential is supplied to a source circuit that supplies the switching control signal. Thus, the third potential can be supplied to the source circuit supplying the switching control signal required for the charge pumping operation enabling high efficiency and low power consumption. It is therefore possible to enable a faster rise after the system power supply turns on without supplying power using only a series regulator, and power can be supplied with high efficiency and low power consumption.
Preferably, the voltage resistance of the source circuit supplying the switching control signal is lower than the voltage resistance of a source circuit supplying the second potential. Thus, faster startup after the system power supply turns on can be achieved and a high efficiency, low power consumption power supply can be maintained even when the source circuit supplying the specific switching control signal requires a power supply with lower potential due to advances in manufacturing processes.
Preferably, the source circuit supplying the switching control signal is a display controller. Thus comprised, the present invention makes it possible to achieve a display apparatus whereby startup is faster after the system power supply turns on and power can be supplied with high efficiency and low power consumption.
In accordance with another aspect of the invention, a method for controlling a power supply circuit that generates a third potential based on a first potential and a second potential is provided. The method comprises the steps of outputting as the third potential an adjusted potential generated by adjusting the second potential; generating a step-down potential by stepping down the second potential using a charge pumping operation based on switching control signal; and switching, based on a switching signal, from outputting the adjusted potential as the third potential to outputting the step-down potential as the third potential.
Another method for controlling a power supply circuit that generates a third potential based on a first potential and a second potential comprises the following steps: outputting as the third potential an adjusted potential generated by adjusting the second potential; generating a step-down potential by stepping down the second potential using a charge pumping operation based on a switching signal; and switching, based on a switching signal, from outputting the adjusted potential to adjusting the step-down potential and outputting the adjusted step-down potential as the third potential.
Other preferable features of the invention include the following.
The switching signal changes after a specific period of time, and the change occurs according to a result of a monitoring circuit monitoring the step-down potential or the third potential.
At least the third potential is supplied to the source circuit supplying the switching control signal, that source circuit having a lower voltage resistance than the voltage resistance of the source circuit supplying the second potential.
The source circuit supplying the specific switching control signal is a display controller.